This invention relates to a new method for the reduction of manganese oxides and is particularly concerned with a method for the solid state reduction of manganese oxides.
Manganese is a commercially important transition metal. Various techniques are used to extract this transition metal from ores.
Maganese coined with other elements is widely distributed in the Earth""s crust The most important ore consists primarily of maganese dioxides in the form of pyrolusite, psilomelane manganite, rhodochrosite or sea nodules. Manganese alloys are conventionally produced through carbothermic reduction of ore and smelting in an electric are furnace. High carbon ferromanganese is also produced in the blast furnace.
The present invention is based on the realisation that manganese oxides can be reduced directly to the carbide where the carbon required for reduction is provided in the form of a gaseous hydrocarbon, for example, methane.
U.S. Pat. No. 4,053,301 describes a process for the direct production of iron carbide from particulate iron oxides reduced using a methane (hydrocarbon)-hydrogen mixture. In the process, fine iron ore is reduced to the metallic state by contacting the ore with hydrogen at a temperature between 595xc2x0 C. and 705xc2x0 C. in a fluidised bed. The reduced iron is then carburised by methane (hydrocarbon). Thus, in the reaction of iron oxide with the methane-hydrogen mixture he products of reduction are iron carbide and H2O, and the overall reaction of the reduction process is presented as:
3FexO+xCH4+(3xe2x88x922x)H2=xFe3C+3H2O
As will be discussed in more detail below, reduction of an oxide by methane (hydrocarbon) in the method of the present invention is fundamentally different from the prior art of iron oxide reductant in that it occurs directly into the carbide phase from the solid material, eg. an ore, with formation of CO.
The process of the present invention may be characterised as pyrometallurgical in nature and based on the use of gaseous reductants, wherein the carbon required for the reduction is supplied from the gas phase.
Accordingly, in a first aspect, the present invention provides a process for the reduction of a manganese oxide to manganese carbide, the process including contacting the manganese oxide in solid form with a gaseous reducing and carburising agent and optionally an inert gas at elevated temperature.
The reducing/carburising gas may be a gaseous hydrocarbon-hydrogen gas mixture. The hydrocarbon may be an alkane, for example, methane, ethane, propane or it may be a mixture of two or more alkanes, or a natural gas can be used, which is optionally cleaned prior to use. Preferably the hydrocarbon is methane. Preferably the hydrocarbon may be present in an amount of about 5-20%, more preferably 7-15%.
Preferably hydrogen in the reducing/carburising gas is present in an amount of about 20 to 95%.
The optional inert carrier gas may be nitrogen or argon. The inert carrier gas may be present in an amount of 0 to 60%.
Preferably the manganese oxide is present in a material having a high gas permeability to allow widespread access of the reducing gas to the oxide phase. Preferably the material treated in the process of the invention has a high porosity, high surface area and is not melted or sintered during the reduction reaction. Preferably the manganese oxide is in particulate form
The material treated in the process of the invention may be an ore containing one or more metal oxides. The ore may be in the form of a pre-concentrate or concentrate. The ore maybe subjected to one or more pre-treatments, for example, concentration by chemical and/or physical means prior to being treated in accordance with the process of the invention. Preferably, the oxide is pre-treated by calcination with hot inert or reducing gases at about 800-1100xc2x0 C. to remove moisture and pre-reduce MnO2 and MnO2O3 to MnO and decompose carbonates.
Preferably the process of the invention is carried out at a temperature high enough for the reduction reaction to take place but not so high as to result in significant melting or sintering of the material being treated.
Preferably the process of the invention is carried out at a temperature in the range of about 1000-1250xc2x0 C., more preferably between 1030-1150xc2x0 C.
The process of the invention may be carried out in any suitable reactor. The reactor may be a fluidised bed reactor or a packed bed reactor. A packed bed may be used if ore particles are prone to sticking. Selection of the most appropriate mode of the process depends on the ore composition, size, and gas composition used.
Preferably the CO is minimised in the reactor atmosphere during the process of the invention. The off-gas from the reactor used to perform the process of the invention may be recycled back to the reactor. Where the off-gas is recycled, it is preferable that CO be removed before recycling to the reaction volume. Some of the gases (reactant gases, off-gases or a separate stream) may be combusted at anytime before, during or after the reactor for the provision of heat either to the reaction volume or to the entering feed.
Hydrogen gas maybe supplied to the reaction to enable the reduction of the iron oxide, present in manganese ore. Silica present in the feed material may partly also be reduced. For example, manganese ores with up to about 12% silica may be treated in accordance with the process of the invention.
The manganese oxide reduction process of the present invention may proceed via the following reaction.
MnO+10/7CH4=1/7Mn7C1+CO+20/7H2
It is readily apparent that this reaction is fundamentally different from the reduction of iron oxide with methane in that the transition metal is converted directly to the carbide phase with the formation of CO gas.
The standard Gibbs free energy of MnO reduction to Mn2C1 is equal to xcex94Gxc2x0=377682xe2x88x92314.44T, J which means that this reaction proceeds spontaneously at temperatures of 1201xc2x0 K. and above when the species are in their standard states. The equilibrium constant for this reaction is log K=10/7 log(PH2/PCH4)+log PCO which is equal to about 10 at 1000xc2x0 C. 100 at 1100xc2x0 C. and 1000 at 1200xc2x0 C. This indicates that MnO reduction to manganese carbide is feasible and has a high extent at 1000-1200xc2x0 C.
Manganese ore, apart from the manganese oxide itself, may contain oxides of iron, silicon and other metals. It is known from literature, that in the process of gas reduction, iron is easily reduced by hydrogen and/or CO gas to the metallic state. Manganese oxide is reduced practically only to its lowest oxidation state MnO.
Examples of materials that cam be treated in the process of the present invention are pure oxides of manganese, Groote Eylandt manganese ores, Wessels manganese ores, and other manganese ores. Preferably the treatment is carried out on particles having a particle size of less than about 2 mm.
The manganese ore is preferably pre-treated with hot inert or reducing gases at about 800-1100xc2x0 C. Superior kinetics may be achieved by pre-treating the ore (oxide) which includes removal of moisture and carbonate decomposition. The calcined solids may then be reduced in a fixed bed reactor or fluidised reactor supplied with an inert gas (such as argon or nitrogen)-hydrogen-methane mixture, and in which the metal oxides are reduced to the carbide. The methane is preferably supplied to the reactor at such a rate and ratio to hydrogen to provide sufficient carbon activity for the reduction of metal oxides and to maintain the desired carbon content in the final product. Hydrogen is introduced to control the activity of carbon in the gas phase.
At a temperature at which the process of this invention is carried out, CH4 is unstable. We have found that by using metastable CH4, a much higher carbon activity in the gas phase can be obtained than that currently available in conventional carbothermic processes.
Accordingly, in a further aspect, the present invention provides a process for the reduction of manganese oxide to manganese carbide, the process including contacting the metal oxide in solid form with a gaseous carburising/reducing agent at an elevated temperature in the presence of an agent that extends the metastability of the gaseous carburising/reducing agent.
Preferably the metastability extending agent is sulphur dissolved in the gas phase.
The gaseous reduction technology of the present invention may provide the following advantages over the standard solid carbothermic reduction routes:
Lower operating temperatures
Ability to process fines
Faster kinetics
Decrease and possible elimination of the coke consumption in the overall metal production route, and therefore, promotion of environmentally friendly technologies, as coke production generates harmful pollutants
Overall decrease in energy consumption.
We believe that faster kinetics is attributed to the better surface contact between the reactants and to the higher carbon activity. Preferably the CO concentration in the reactor atmosphere is minimised because CO reduces the kinetics and extent of reduction.
Since the reduction of ore occurs at the ore/gas interface, greater porosity of the oxide material results in greater contact area and kinetics.
The manganese carbide produced by the process in accordance with the invention may be used to produce alloys, for example, by smelting of metal carbide, or directly used in steelmaking.
The present invention extends to the manganese carbide product produced by the process of the invention. The invention also extends to metals and alloys produced from manganese carbides formed by the process of the invention.
In order that the invention may be more readily understood we provide the following non-liming embodiments.